Hashcash, the proof of work

In May 1997, Adam Back, a British cryptographer active on the cypherpunks list, proposes Hashcash. The original goal has nothing to do with money: it is about fighting spam. The idea is to force the sender of an email to solve a small cryptographic puzzle, to find a value whose hash begins with a certain number of zeros. The computation takes a few seconds for a single email, but becomes ruinous for anyone wanting to send millions of them.

This mechanism is called proof of work. Its magical property: the puzzle is costly to solve but instant to verify. Anyone can confirm in a single operation that the work was actually done, without trusting whoever did it.

Hashcash thus introduces an objective digital scarcity: each token represents a quantity of electricity and computing time genuinely spent, which no authority needs to certify. The ledger and the circulation are still missing, but the anchoring of value in the physical world is there.

Eleven years later, the Bitcoin whitepaper cites Hashcash explicitly (reference number 6) and makes it the heart of its consensus: miners compete over proofs of work, and the longest chain, the one that accumulates the most work, is authoritative. Adam Back is also one of the very first people Satoshi contacts by email, in August 2008, even before the whitepaper is published. Read the Hashcash paper (PDF, 2002), whose form the Bitcoin whitepaper will borrow. Stylometric comparison of the two texts.